The cooperation between base stations is an important means to mitigate inter-cell-interference in cellular systems and is being intensively discussed in the fourth generation of wireless communication system standardizations. It is important for the specifications to enable flexible CoMP (Coordinate Multiple Points) operations, such as JT (joint transmission), coordinated beamforming, and dynamic point selection, etc.
It is noted that different CoMP operations require different CQI (Channel Quality Indicator) calculation assumptions. For example, for JT transmission, a UE (user equipment) would assume the signal power is from multiple cells (transmission points) that transmit PDSCHs (Physical Downlink Shared Channels) to the UE, while other cells are interfering cells. But, for CB (coordinated beamforming) transmission, the UE would assume the signal power is from only one cell, while other cells are interfering cells. In general, the signal power can be measured based on CSI-RSs (Channel Status Information Reference Signals) from corresponding cells, which is similar to current techniques (3GPP (3rd Generation Partnership Project) Rel-8/Rel-10). However, the current technique on interference measurement does not work well, because it can measure only the overall interference power (interference from all cells except the serving cell). In general, in Rel-11 and after, it would preferred to be able to measure interference from each TP (transmission point) to better cope with flexible CoMP operations. Therefore, a new interference measurement mechanism is required for base station cooperation.
One method to enable per-TP interference measurement is to have each TP to transmit its own reference signal for the purpose of interference measurement. Those reference signals may be overlapped in time and frequency domain to enable easy spatial reuse of the time-frequency resources. To reduce the effort in standardization, it would be an attractive choice to reuse the CSI-RS configuration in Rel-8/Rel-10.
Reusing the CSI-RS configuration includes two folds of meanings: the first is to reuse the CSI-RS time and frequency positions, and the second is to reuse the CSI-RS OCCs (Orthogonal Cover Codes) and scrambling sequences. To avoid potential impact to legacy UEs, it is strongly preferred to reuse the CSI-RS time and frequency positions. However, reusing OCCs and scrambling sequences have some drawback in heterogeneous networks, therefore a new OCC and scrambling may be further investigated. It is noted that the revised OCC and scrambling does not reduce legacy UE performance.
The RS (Reference Signal) reusing CSI-RS time and frequency positions for interference measurement purposes may be called IM-RS (Interference Measurement Reference Signal). FIG. 1 shows an example of IM-RS configurations from different TPs. In FIG. 1, IM-RSs from three different TPs, i.e. TP1, TP2 and TP3, are transmitted on the same time and frequency positions (indicated by boxes filled in black in FIG. 1) by different descrambling. That is, different TPs transmit one port IM-RSs based on the same IM-RS configuration (time and frequency positions) but scrambled differently. The UE calculates respective interference powers after descrambling.
The problem of the previous OCC and scrambling in a heterogeneous network is as follows. In a heterogeneous network, a UE may be associated with a TP that has relatively lower receiving power for traffic offloading purposes. More specifically, a UE may be associated with a LPN (Low Power Node) while the receiving power from a macro node is much higher than the power from the LPN. In this case, when the UE intends to estimate the interference power from another LPN via descrambling the related port, the residual interference from the macro node may be much higher than the interference power from the another LPN, which implies very inaccurate interference power estimation of the another LPN. The impact is more severe for subband CQI calculations because of shorter scrambling sequences.
Now, detailed analysis of an exemplary heterogeneous network is given with reference to FIG. 2 and FIG. 3. FIG. 2 shows an example of a heterogeneous deployment, and FIG. 3 shows an example of IM-RS scrambling configuration for the case of FIG. 2. In FIG. 2, the UE desires to receive data from LPN1 functioning as the serving transmission point while LPN2, LPN3 and macro node are considered as interfering transmission points. For example, the macro node has a transmitter power of 46 dBm which is much higher than those of LPN1, LPN2 and LPN3. As shown in FIG. 3, IM-RSs for LPN1, LPN2, LPN3 and macro node are configured based on the previous OCC and scrambling such as in Rel-10. Specifically, four layers of IM-RSs are multiplexed on the same time and frequency resources by two length-2 OCCs [1, 1], [1, −1] and two sets of scrambling sequences [S1, S2], [S3, S4]. That is, the macro node and LPN2 are assigned with the OCC [1, 1] while LPN1 and LPN3 are assigned with the OCC [1, −1]. The sequences S1, S2 are respectively multiplied to the two symbols of the OCCs for both the macro node and LPN1 while the sequences S3, S4 which are different from the sequences S1, S2 are respectively multiplied to the two symbols of the OCCs for both LPN2 and LPN3. Here, the sequences S1 and S2 are independent from each other and so do the sequence S3 and S4. In such a case, the CQI calculation at UE needs to know the interference from each of the interfering transmission points LPN2, LPN3 and macro node. Taking LPN2 as an example, the measured interference from LPN2 is calculated as follows by the equation (1).
                                          P            ^                                LPN            ⁢                                                  ⁢            2                          =                              P                          LPN              ⁢                                                          ⁢              2                                +                                                                                          s                    3                    *                                    ⁢                                      s                    1                                                  +                                                      s                    4                    *                                    ⁢                                      s                    2                                                              2                        ·                          P              macro                                +                      …            ⁢                                                  ⁢                                                                                                      s                      3                      *                                        ⁢                                          s                      3                                                        -                                                            s                      4                      *                                        ⁢                                          s                      4                                                                      2                            ·                              P                                  LPN                  ⁢                                                                          ⁢                  3                                                              +                                                                                          s                    3                    *                                    ⁢                                      s                    1                                                  -                                                      s                    4                    *                                    ⁢                                      s                    2                                                              2                        ·                          P                              LPN                ⁢                                                                  ⁢                1                                                                        (        1        )            where PLPN1 denotes the power the UE receives from LPN1 and the symbol “*” indicates conjugate.
                    s        3        *            ⁢              s        3              +                  s        4        *            ⁢              s        4              2in the equation (1) is zero, but
                              s          3          *                ⁢                  s          1                    +                        s          4          *                ⁢                  s          2                      2    ⁢          ⁢  and  ⁢          ⁢                              s          3          *                ⁢                  s          1                    +                        s          4          *                ⁢                  s          2                      2  are non-zero. Thus, the interference from LPN3 is removed well but those from LPN1 and macro node are remained to impact the measured interference from LPN2. Especially, since Pmacro is much higher than LLPN2, the interference estimation for LPN2 is very inaccurate.
Therefore, how to accurately estimate the interference power from one or multiple transmission points which may or may not have the strongest receiving power is required to be solved.